Fix stack deletion when orig-base is present.

The introduction of the orig-base file makes the stack deletion fail
to remove the stack's directory, leaving the stack in a zombie state,
where attempting to recreate it was refused because of the directory
already existing, but attempting to delete it failed since the applied
and unapplied files were not found.

Signed-off-by: Yann Dirson <ydirson@altern.org>